$ D = \left[\begin{array}{rrr}1 & 2 & -1\end{array}\right]$ $ E = \left[\begin{array}{rr}1 & 2 \\ 1 & 4\end{array}\right]$ Is $ D E$ defined?
In order for multiplication of two matrices to be defined, the two inner dimensions must be equal. If the two matrices have dimensions $( m \times  n)$ and $( p \times q)$ , then $ n$ (number of columns in the first matrix) must equal $ p$ (number of rows in the second matrix) for their product to be defined. How many columns does the first matrix, $ D$ , have? How many rows does the second matrix, $ E$ , have? Since $ D$ has a different number of columns (3) than $ E$ has rows (2), $ D E$ is not defined.